Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers, and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of secondary batteries capable of repeatedly charging and discharging has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available, and among them, lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. However, additional research is still urgently needed to improve the relatively low capacities and in particular, insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, safety regulations strictly prohibit the dangers (such as fire and smoke) of electrochemical devices. In connection with the safety characteristics of an electrochemical device, overheating of the electrochemical device may cause thermal runaway or puncture of a separator may pose an increased risk of explosion. In particular, a porous polyolefin-based substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinkage behaviors at a temperature of 100° C. or higher due to its material characteristics and production processes including elongation, causing short circuits between a cathode and an anode.
Various proposals have been made to solve the above safety problems of electrochemical devices. For example, Korean Patent Publication No. 10-2009-0130885 discloses a multilayer porous film structure having a porous layer containing an inorganic filler by applying a methacrylic acid ester-acrylic acid ester copolymer as a binder. However, the binder of the acryl-based copolymer composition disclosed in this patent does not provide a practical alternative. In particular, the disclosed composition does not guarantee reliability in durability since when the thickness of a coating is 4 μm or less based on a cross section, a delamination phenomenon of a coating layer caused by swelling of an electrolyte solution occurs due to insufficient thermal and physical stability, and in particular, low resistance against an electrolyte solution.
Meanwhile, Korean Patent No. 10-0923375 discloses a separator manufactured by supporting a thermally crosslinkable adhesive composition on a porous substrate, the thermally crosslinkable adhesive composition including polyfunctional isocyanate and a reactive polymer having a functional group capable of reacting with an isocyanate group of the polyfunctional isocyanate. However, there is a limitation in production efficiency since the composition disclosed in this patent takes 7 days to cure at 50° C., and so a large scale of additional facility investment and time for curing is required to produce.